This invention relates to an unsaturated polyester resin composition, and particularly to an unsaturated polyester resin composition that is moldable under low pressure and low temperature conditions. These compositions exhibit extended shelf lives, good flowability, excellent handling properties, good curing characteristics and stable material viscosity.
Unsaturated polyester resin compositions are well known for use in sheet molding compounds from which a variety of articles can be molded. The unsaturated polyester resin compositions generally include an unsaturated polyester component, one or more thermoplastic low profile additives, and at least one olefinically unsaturated monomer which copolymerizes with the unsaturated polyester. The resin composition may include an organic peroxide for catalyzing the reaction, a stabilizer and a filler, and generally further includes one or more fibrous reinforcement materials, such as chopped glass or textile fibers. Due to their excellent mechanical strength, heat resistance, water resistance and chemical resistance, fiber-reinforced plastics produced from these unsaturated polyester resin compositions are used in a number of diverse applications, such as molded bathtubs, water tanks, or load-bearing structural elements. In the transportation industry, fiber-reinforced plastics are used to produce oil pans, heat shields, rocker covers, grill opening panels, and exterior body panels, and other components. The degree of smoothness, affinity for paint and quality of other surface characteristics desired generally depends upon the application in which the fiber-reinforced plastics are employed. For example, the quality of the surface characteristics may range from "Class A" typically used for the exterior surfaces of automobiles to surface smoothness acceptable in typical general purpose applications.
There are various methods known in the art for molding fiber-reinforced plastics. Among such methods are open mold techniques and closed mold techniques. Open mold techniques include the hand lay up method and the spray-up method. Open mold techniques are generally used when forming large-sized parts such as boat hulls, van tops, fenders, or similar articles. While open mold techniques allow lower temperatures and pressures, there are many problems associated with the techniques. Additionally, for example, in cold press molding, the molding composition is poured in cold by hand. As the compound cures, the edges must be trimmed around the mold, resulting in wasted raw materials. Further, the molder is exposed to dispersion of styrene and scattered fibers throughout the process and the molded article has a poor surface quality. Other low temperature techniques, such as resin transfer molding, require a long cycle time of at least 15 minutes to one hour in order to form a single part or component.
Closed mold techniques, such as compression molding methods, are generally used to mold articles from unsaturated polyester based sheet molding compounds (SMC) or thick molding compounds (TMC). A hydraulic press provides the high pressure and elevated temperatures required to form the article. Since it is a closed system, closed molding techniques are free of the dispersion and scattering of styrene and fiberglass typically associated with open mold techniques. However, the steel molds required to withstand the high pressures and temperatures are expensive. Therefore, a large capital investment is required for the press and molds.
Accordingly, there is a need for a resin composition which can be molded at low temperatures and pressures in less expensive equipment. There also is a need for a resin composition which produces molded articles having surface characteristics and mechanical properties similar to articles molded with conventional high temperature high pressure techniques. There is a further need for a low temperature low pressure resin composition which significantly reduces the cycle time required to mold an article.